In photovoltaic (PV) power generation systems, the operation point of panels can be driven to the best possible operation point, for example, to a point in which a maximum amount of power can be extracted from the panels. The most common maximum power point tracking (MPPT) method is the perturb & observe (P&O) method and its variations. In this method, the voltage or current reference can be constantly changed, and the produced change in generated power can be determined. On the basis of the change of power and the change of the reference value (voltage or current), it can be determined whether the reference should be decreased or increased for increasing the amount of extracted power.
The variations of a P&O algorithm include changing the sampling frequency or the step size of the reference on the basis of changes of the generated power and the given reference. Further modifications include using multiple samples for determining the change of power due to a changed reference.
The modifications of a P&O scheme can relate to increasing the accuracy of the scheme during changes in irradiation. For example, it is a known issue with P&O based algorithms that under constantly changing conditions the algorithms are not able to find the maximum power point.
FIG. 1 shows the operation of a P&O algorithm during a linear change of irradiation. During increasing irradiation at a time interval of 2 seconds to 9 seconds, the voltage of the panel system can be decreased during the whole interval, as can be seen in the lower plot of FIG. 1. As a consequence, the extracted power does not follow the linearly increasing irradiation as it should, as can be seen in the upper plot of FIG. 1. As such, an MPP algorithm cannot keep the operation point at a maximum power point. For example, at a time interval of 11 seconds to 19 seconds, the irradiation can be linearly decreased. The voltage of the panel system (lower plot) can be changed back and forth and can thus be kept almost at the same level. The extracted power (upper plot) cannot be at its maximum during the change.
The operation of the P&O method is disturbed by the changing irradiation since the observed change in power results from both the changed irradiation and the changed reference voltage or current. The method itself cannot separate the origin of the power change and therefore can arrive at wrong conclusions about the direction of the next voltage or current reference.
The maximum power point voltage of the panel does not change much even when the irradiation level changes from 500 W/m2 to 1000 W/m2. Therefore, the correct operation of the tracker can be affected and the tracker should change the voltage of the panel system in a controlled manner.